A mare comes in from the field one afternoon with an eye that is red, irritated and tearing. Her owners chalk it up to flies and buy a fly mask. While the mask helps keep insects away from her eyes, the mare continues to suffer with painfully puffy, weepy eyes. By the end of the week, they notice that the eye looks cloudy and the mare is reluctant to come out of her stall into the sunlight. Alarmed, they call the veterinarian who concludes the examination with a worrisome diagnosis: uveitis, which is more formally known as periodic ophthalmia, or ERU, for equine recurrent uveitis.